1. Technical Field
The present invention relates to a polishing tool and a polishing method suitable for polishing an optical surface of an optical element, such as a lens, and more particularly, to an elastic polishing tool and a polishing method suitable for polishing an aspherical surface.
2. Related Art
In the related art, a concave optical surface of a spectacle lens, such as a spherical surface, a rotation-symmetry aspherical surface, a toric surface, or a progressive surface, is formed by cutting. Then, the formed optical surface is mirror-polished. A rub polishing method using a rigid polishing plate is generally used to mirror-polish a simple curved surface, such as a spherical surface or a toric surface. Since the mirror polishing method using the polishing plate transfers the surface shape of the polishing plate onto a surface to be polished, it is necessary to prepare polishing plates equal to the number of surface shapes corresponding to lens prescriptions. There are several thousands of types of polishing plates.
Further, it is difficult to polish a free curved surface, such as a progressive surface having a complicated shape, using the rigid polishing plate. Therefore, an elastic polishing tool is generally used to polish the free curved surface.
A polishing method using a balloon-type polishing tool is disclosed in JP-A-2003-275949 as the polishing method using the elastic polishing tool. In this polishing method, pressure air is pumped into the balloon-type polishing tool to inflate it, and the pressure of air is adjusted to change the curvature of the polishing tool according to the shape of a curved surface to be polished, thereby performing polishing. Therefore, it is possible to cope with a plurality of polishing surfaces having different curved surface shapes, which makes it possible to polish a plurality of surfaces having different shapes by using only one balloon-type polishing tool.
Further, there is a partial polishing method using a dome-shaped elastic polishing tool which comes into contact with a portion of a surface to be polished (for example, see JP-A-2000-317797). The polishing method is the method of polishing the entire surface to be polished by connecting local polished areas polished by a small elastic polishing tool.
In the related art, a polished spectacle lens to be fitted to a spectacle frame generally has a circular shape. However, in recent years, with a reduction in the thickness of a spectacle lens, a polishing process has been performed to obtain the smallest thickness at the center of the lens on the basis of spectacle frame data and a lens prescription. Therefore, the production of a spectacle lens having a substantially elliptical (aspherical) outward shape and a sharp edge tends to increase.
However, in the polishing method using the balloon-type elastic polishing tool disclosed in JP-A-2003-275949, since the balloon-type elastic polishing tool comes into contact with the entire surface of a polishing target to polish it, it is possible to polish the polishing target in a short time. However, when a lens having a substantially elliptical outward shape and a sharp edge is polished, the edge of the lens is dug into a polishing pad adhered to a balloon polishing portion, which causes the polishing pad to be peeled off or the polished lens and the polishing tool to be damaged. When the internal pressure of the balloon polishing portion is raised in order to solve this problem, flexibility is deteriorated, and shape trackability is degraded. As a result, an unpolished region or polishing irregularity may occur.
Meanwhile, in the partial polishing method using the elastic polishing tool disclosed in JP-A-2000-317797, polishing is performed while moving the elastic polishing tool on the entire surface a material to be polished. Therefore, since an elastic body coming into contact with a surface to be polished can change the shape thereof according to the shape of the surface to be polished, polishing irregularity hardly occurs in a polished surface of a lens having a circular outward shape. However, when a lens having a substantially elliptical outward shape and a sharp edge is polished, the elastic polishing tool intermittently comes into contact with the lens. Therefore, the edge of the lens is dug into the polishing pad, which causes the polishing pad to be peeled off, or causes the polished lens and the polishing tool to be damaged.